Fibrous alumina monohydrate and its production



J. BUGOSH 3 Sheets-Sheet 1 FlG.2

INYENTOR JOHN BuGosH l BY @gw/*ff TTORNEY Dec. l, 1959 FIBROUS ALUMINA MONOHYDRATE AND ITS PRODUCTION Filed Dec. 29. 195s F I G'. 1

4 m, G. F f 5 ul l m, l G

FIBROUS ALUMINA MONOHYDRATE AND ITS PRODUCTION Filed Dee. 29. 195s J. BUGOSH Dec. 1, 1959 3 Sheets-Sheet 2 FIGT TIME

TIME

TIME

50 0F THE FIBROUS TIME 9 INVENTOR JOHN BUGOSH ATTORNEY Dec. 1, 1959.l J. BuGosH 2,915,475

FIBROUS ALUMINA MONOHYDRATE AND ITS PRODUCTION Filed Dec. 29, 1958 3 Sheets-Sheet 3 UL U4, INVENTOR JOHN BUGOSH Qnited States Patent O FIBROUS'ALUMINA MoNoHYDRATE AND ITS l PRODUCTION i Application December 29, 1958, Serial No. 783,602 16 Claims. (Cl. 252-313) This invention relates to fibrous alumina monohydrate characterized by having the boehmite crystal lattice.

In the drawings:

Figure 1 is a pen-and-ink drawing made fromia photomlcrograph at 25,000 diameters magnification of fibers of the invention made up of fibrils disposed parallel to the length of thefiber;

Figure 2 is a similar showing of a product in which the fibrils are relatively non-aggregated and are not oriented; f. Figure 3 iis-a similar showing at 50,000 diameters magmfication of a product in which the fibrils are more aggregated, in an orientedfashion, than in Figure 2. The black, circular objects in this and in following photomicrograplis are polystyrene latex particles having a diameter of 280 millimicrons;

lFigure 4 shows, at 18,000 diameters, a modified product prepared by azeotropically removing water from a product like that of Figure 2;

Figure 5 is a representation of the infrared spectra of products of the invention; f

Figures 6, 7, 8, 9, and 10 are curves required in consecutive steps for determining the time, 0, required to depolymerize half of a product of the invention in acid; Figure 11 is a pen-and-ink drawing made from a photomicrograph at 26,000 diameters `magnification of the product of Example 23;

Figure 12 is a similar showing of the product of Example 24 at 18,000 diameters;

Figure 13 is a similar showing of the product of Example 26 at 18,000 diameters; and

Figure 14 is a similar showing of the product of Example 27 at 35,000 diameters.

y In processes of the invention broadly, an aqueous dispersion of alumina is prepared and this is heated in the presence of a strong acid radical such as the chloride ion of` hydrochloric acid. The concentration of the alumina and the acid radical are adjusted as will be further described hereinafter, and it is preferred that the ratio of acid radical to alumina be maintained within limits further to be described.

Products of the invention can of course be produced .inV other ways as by heating in the presence of acetic or formic acids as will be hereinafter described.

The aqueous dispersion of valumina is heated according to the invention and as a result of this heating under vthe conditions of the invention, the alumina forms fibrous alumina monohyrdate having the boehmite crystal lattice. Fibrous alumina monohydrate will sometimes be referred to hereafter as fibrous boehrnite because of the similarity of its crystal lattice to that of naturally occurring boehmite.

Preferred processes of the invention effect removal of 'forms of lalumina other than brous alumina monohydrate having the boehmite crystal lattice or convert them Ito such alumina. Acid radicals can also be removed from the product. i

Depending upon the specific process conditions selected,

ICC

the fibrous alumina product will be in the form of fibrils which have one or more dimensions in the colloidal range. Such fibrils can form aggregates of large fibers made up of assemblies of fibrils disposed parallel to the length of' the fibers.

A preferred group of products of the invention are colloidal sols of the fibrils or fibers n water or organic media. The fibrils or aggregates can be used in the form of sols or dispersions in-water or other liquids, or they can be dried from the sols or dispersions and used as dried products.

In a preferred aspect of the invention the fibrils or aggregates of fibrils are driedy by removing water from them inthe presence of an organic liquid that is partially water-miscible to produce liuffy products of high specific surface area.- If the organic liquid chosen is an alcohol, surface esterification of the boehmitecan be eected. A preferred dry product is an easily redispersible powder which will give a stable sol of fibrous alumina particles almost indistinguishable from the sol from which the powder was prepared. A particularly preferred dry product can be made by spray drying an aqua-sol of the invention in a conventional manner. Still further product and process modifications will be described hereafter.

THE ALUMINA STARTING MATERIAL hydration, and particle size that the time, 6, required to depolymerize half of a sample in acid into aluminum ions is kless than l0 minutes and is preferably less than one minute.

The k0 value is useful in characterizing the alumina to be used in processes of the invention and in defining the process conditions specifically applicable to each such alumina. The@ value is also useful in characterizing products of the invention. f

The 0 value for a particular alumina starting material is quantitatively expressed as the time in minutes required for the depolymerization of half of'a sample of the alumina in the presence of excess hydrochloricv acid at a temperature of 98 C. The 0 value of an alumina starting material can be determined as follows. An amount of the alumina sample equivalent to 4.8 lgrams of A12O3-is weighed out. One hundred and eleven milliliters of 5.0 N HCl is heated to 98 C. The mole ratio of acid to alumina is thus 12:1. Distilled water sufiicient with the alumina and acid to make a total of 200 grams is measured out.A The water is added to the alumina sample andthe mixture is heated to 98 C. The diluted alumina sample and acid are mixed, stirred and transferred to a stoppered bottle and placed in a controlled temperature bath held at 98 C. If the alumina sample is a sol or dispersion which is not stable or which is not readily prepared at a concentration such that 4.8 grams as Al2O3 can be contained in the amount of water involved in this technique, then the amounts of acid andalumina can be reduced but maintained in the ratio of 12:1 moles as above.

Ten gram samples are taken at intervals. Each is 4Onesample is taken immediately and others are taken Y at measured time intervals of about, say, three to tive minutes. If it is found that the sample is rapidly depolymerized then a special effort can be made to effect titration as soon 'as possible after adding the acid to the alumina and as frequently as possible thereafter. If the sample is more slowly depolyrnerized then the time intervals can be extended.

The results of the titration are interpreted as in the determination of the concentration of a weak acid in the presence of a stronger one. As the base lis added to the system any excess of the unreacted hydrochloric acid is first "neutralized with the base until the pH rises rather rapidly to about pH 3.5. The aluminum chloride, or other aluminum salt, acts as a buifer and the pH does not rise further until `it has been neutralized, The titration is continued until the IpH rises to about 8. More precisely the inflection `point is reached at 7.25. The moles of sodium hydroxide used to eiiect neutralization between pH 3.5 and 7.25 is then divided by 3 to give moles of aluminum ion in the system. This type of titration is discussed in greater detail in Treadwell et al. HelvetiaChim. Acta (1932), 980.

Instead of determiningfthe concentration of depolymerized alumina ions by titration one can instead use other standard methods for determining aluminum ion concentration in the presence of polymerized alumina.

For example, colorometric determinations, polarographic determinations, or gravimetrie determinations can be used. The procedure above given, however, is rapid, accurate, and convenient when the sample does not lcontain interfering substances like acetic acid or iron, titratable inthe same pH range,

After the amount of alumina in each of the samples taken `has been determined as by titration, these quantities can be plotted against time. The time required t0 effect depolymerization of half of the alumina can then readily be picked from the resulting graph. As has been noted briey above, if the time intervals were not well selected in the rst instance then a new rset `of samples should be taken over shorter periods or over longer periods as required to give a satisfactory plot. The method of plotting such data and its interpretation is further described hereafter in connection with determining 0 for products of the invention.

The alumina starting materials are rapidly depolymerizable in acid, having a 6 value of ten minutes or less or preferably one minute or less.

In forming dispersions of alumina in water to be used in processes of the invention, there can be used as starting materials basic aluminum chloride, basic aluminum nitrate, aluminum hydroxide, alumina gels, or colloidal solutions of alumina. `In all of these, alumina `is present in the aqueous system in a dispersed condition. Aluminum is associated with oxygen and is probably in some degree of hydration. In the aqueous dispersions employed in the processes of the invention, -it will be associated, therefore, with oxygen, with hydroxyl, with water, and with an acidradical such as chloride. Y

It is not feasible to determine the precise degree or character of hydration of the alumina vor the mode of combination of the oxygen, the acid radical and water in the system. It is nevertheless the fact that the aluminum present is undoubtedly combined in some manner with Voxygen and upon evaporation and ignition of the solution, a residue of A1202 is obtained. In aqueous solutions or dispersions suitable for use according to the invention, it will accordingly be possible to dry the solution, ignite the residue, and determine the A1203 con tent. Thus, in referring to alumina in the aqueous dispersions used, it will be understood that the term signifies the A1203 content as so determined and not that the aluminumfin the dispersions is necessarily present as the specific compound A1202.

Therefore, in speaking 'of the alumina as dispersed in an aqueous system, it will be understood that this term is used to include solutions such as those of a basic aluminum chloride, colloidal dispersions, or colloidal solutions such as various aluminum hydroxide sols, or suspensions of highly hydrated alumina such as precipitated aluminum hydroxide,

Considering typical materials with a 0 less than 10 which serve as a source for alumina in the processes of the invention, there can lbe mentioned solutions of aluminum chloridewhich have been partly neutralized with a base to form a basic aluminum chloride. Simi'-` lar solutions can be made as shown in Huehn andHa'ufe U.S. Patent, 2,196,016, April 2, 1940, by dissolving metallic aluminum in aqueous solutions of aluminum nitrate or aluminum halides under controlled conditions. There can also be used as starting materials basic aluminum nitrates prepared according to U.S. Patent 2,127,504 and German Patent 444,517. It is observed that nitrates have the advantage over chlorides that they can be treated `in stainless steel equipment without undue corrosion.

A suitable basic aluminum nitrate is one prepared by heating an aluminum nitrate nona hydrate melt to C. This yields a basic aluminum nitrate Al(OH)2NO3 as shown in the above U.S. Patent 2,127,504. This product is then heated further to remove Voxides of nitro-` gen and to obtain `a basic aluminum nitrate of higher aluminum to nitrate ratio. This ratio would bein the range of A12031NO3 of 1:1 to 4:1. A particularly preferred range of AI2O3:NO3 is from 1.2:'1 to 2.0`:1.`

`Freshly precipitated aluminum hydroxide which been carefully washed to remove salts will also` serve to form dispersions of alumina. This can be dispersed mechanically in water and a strong acid added to obtain the conditions which are preferred for use in processes of the invention.

`An alumina gel well suited vfor use in processes of the invention can be prepared by precipitating a basic aluminum carbonate by the addition of a sodium carbonate solution to a solution of an aluminum salt. The basic aluminum carbonate gel thus prepared `contains carbon dioxide which can be displaced by heating or, more easily, by heating after the addition of a small amount of the acid to be used in the 'subsequent process. Thus there can be used a small `amount of hydrochloric acid, nitric acid, or another `of the acids hereafter disclosed for use in processes of the invention. A discussion of methods of characterization of gels of various types is found in E. Calvert et al;, Soc. Chim. de France, Bull. (5) 20, 99-108 (1953).

Sols of amorphous alumina suitable for use in processes of the invention can also be prepared by theelectrodialysis of solutions of aluminum nitrate, for example, to produce aqueous dispersions of alumina having the desired quantity of nitrate radicals. Sols can be used which have been prepared by the ion exchange of aluminum saltswith ion exchangers.

In the above discussion it has been assumed thata particular alumina is homogeneous and contains `but one form of alumina. It will, of course, be understood `that mixtures of alumina `containing more` or less of various types of either crystalline or amorphous alumina can be present in a single sample. 1 d l In carrying out processes ofthe invention, the alumina as an aqueous dispersion oras 'a 'sol in ywater Vis preferably free from impurities and compounds other thantl'ieY alumina and the acid radical which will be described in more detail hereafter. Soluble salts such as sodium tical. of dispersions which do contain such salts.

ticularlyimportant that there be at the most only small amounts of'compounds of silicon, boron, and molybdenum as impurities. All of these tend to block fiber lformation. This tendency is especially pronounced if a boron compound is copolymerized-with the alumina. Small amounts of silica will normally be present but the level should be kept as low as practicable. To this end it will often be desirable, especially when operating at high temperatures say above 250 C., to operate processes of the invention in equipment which is not lined with glass or other siliceous material. Other soluble salts can aid dispersibility of dry products or they may be wanted in the product for other reasons and can be left in, or added to, the starting materials.

THE PROCESS CONDITIONS In processes of the invention anaqueous, acidic dispersion of alumina is heated until fibrous alumina monohydrate having the boehmite crystal lattice is formed.

When fibrous alumina is prepared according to the invention using as a starting material an alumina with a' low degree of polymerization such as basic aluminum chloride, the reaction is effected by the joining together of small units. This is a polymerization-type of reaction.

' If the alumina. which is used as a starting material is highly polymerzed the reaction may proceed first by a depolymerization to lower molecular weight units followed by a repolymerization to fibrous alumina.

However, the formation of fibrous alumina monohydrate having the boehmite crystal lattice may be the result of a localized rearrangement of the already polymerized material.

In any even the first step is to form a suspensionin water of the alumina starting material. If the material is a sol then of course it is already suspended. If it is water soluble at the pH of the reaction then it is simply dissolved in water which contains the acid to be used in the process.

If the alumina is not readily soluble in the acidic system to be used in the process it can simply be suspended by stirring. Generally, if a highly polymerzed alumina is used the particles should be of such a size that they will pass through a 100 mesh screen, or preferably at a size to pass through a 300 mesh screen.` It will be understood that the suitability of a particular alumina as to particle size and character can be determined, as already shown, by the value for the material.

The,A process conditions which principally affect the character of the product obtained are:

v(l) Acids used.

(2) Concentration of A1303,

A( 3) Concentration of acid,

(4) Ratio of A1303 to acid,

(5) Operating temperature, and

(6) Temperature-time relation.

1. Acids used The aqueous dispersions treated according to process of the yinvention contain an acid radical. By this, it is of course-meant that there is present in the solution an acidradical titratable with sodium hydroxide. This excludes, for example, the chloride in salts such as sodium chloride and similar acid radicals in other neutral salts which may be present in the dispersion. The acid radicals can be present, for example, in solutions of basic aluminum chloride. In such solutions, the chloride ions are titratable with alkali. Thus a sample of the solution can be titra'tedl with a standard solution of sodium hydroxide until the end point is reached at a pH of 7.25. The titration value gives a measure of the concentration of acid radicals present in the solution and will, of course, not include anions of neutral salts such as lthose of sodium chloride..

2. Concentration of A1303 The concentration of alumina as A1303 can be widely varied without greatly modifying the character of the products produced. The upper limit on the alumina` concentration is fixed by the excessive, irreversible aggregation of the product. If a concentration of more than about 1.6 molar A1303 is used at temperatures below about 250 C., the products produced are so badlyy and irreversibly aggregated that they are not preferredproducts of this invention.y The tendency toward aggregation depends, however, on the temperature of the reaction as well as upon the concentration of the alumina used. In general, if temperatures considerably above 250 C. are used, it is then possible to produce preferred products of thisv invention at alumina concentrations as high as 3 molar.

Below an A1303' concentration of about 0.05 lnlole per liter, products of the invention are not obtained. The molarity of A1303 should not exceed about 1.5 for the preferred processes of the invention.

Alternatively it may be stated with respect to preferred processes of the invention that theY concentration of alumina can be varied between 0.05 and 1.6 moles per liter' when temperatures below 250 C. are used. The concentrations can be varied between 1.6 and 3.0 moles per liter when temperatures above 250 C. are used. It is preferred to operate below 250 C.

3. Concentration of acid The upper limit of the acid anion concentration should not greatly exceed 4.2 molar. The lower limit of the acid radical concentration kcannot fall below a value of approximately 0.05 molar without limiting the eiciency of the process. The hydrogen ion probably functions as a catalyst in the production of fibrous alumina monohydrate having the boehmite crystal lattice and it may be that at a concentration below about 0.05 molar there is too little acid present to have enough value as a catalyst for the reaction to proceed at an appreciable rate. n

In referring to acid radical concentration or acid anion concentration reference is made to the discussion earlier in the application to the definition. of acid radical as that titratable with sodium hydroxide.

For aluminas as above generally described the acid radical concentration in the aqueous dispersion should have at least a molarity of 0.05 of one or more of the acid radicals described. yIn preferred processes yof the invention the molarity will notl greatly exceed about 4. It is still more preferredto operatefwith a molarity for the acid radical no higher than about l. The term molarity is used herein, of course, to mean the moles of acid radical per liter of aqueous dispersion.

4. Ratio of alumina to acid` The relative proportions of alumina to acid radical can be considerably varied and it will usually be preferred to use an aqueous dispersion in which the molarity of the acid radical is from aboutone-fourth the A1303 molarity to a molarity equal to that of the A1303 lplus g2. Inmost words,`the molarity of the acid radical is rom:

where (A1303) is the alumina molarity in thel aqueous dispersion calculated on the basis of A1303# I. .Moi-ej ,specitically it. `iis;.preferredjto.`r operate. withz a y :peratures used Ithat it wilt 1in general be desired toA am new f l 5 .sans ath-@aanrader famo-S, wea @$121031 2 S large? amounts otwealtgacids 'such as facetic -andgtorrnic i i aci :isi as' hereinafterdescribed'. Suchwealt acids :do not u .completely and angexeess does not trouble the resei on.; "Thus the molarity of acid radical canextcnd :11pi /ariyto 4S( 203i) .Greven as .highfasgsay {A1203} yor .eveuhishen 1 l1'.5'..trkei-ating temperaturas@ n l boah-mite crystal lattice;

understood'that'ths is notneccssarily so. i Frequently it l vvill. befouud desirable to charge ari-alumina .sol intoan lautociave andfraise-itstemperaturefoverfaperiodof time. g f f Log lis the logarithm to the base f1@ 'of the expression. 'f

l It will' benoted'fthatwhilef Athe conditions recited are preferred :for the strong acids,r it is permissible to use :'Generaliy.theflowest-temperature .at which the yprocess l can be conducted without excessively long time .is .120 C. l l lemperaturesin excess Vof .about 250 C. are not pirev l tened. Above 400 C. theE bofehmiteforrn 'is notentirely jstjaole and other crystailjine'forms lwhich are imore stable beginrto appear. j, j 'i l j While temperature has. been. discussed Eas .constant throug'hout proce'ssjes ofgthe invention. 1

f The temperature' will usualvstart near-room temperature;

f temperaturesabove describedr referto ,the maximum tern-l perature reached and this is .ordinarily the ,temperaturek i atl which the charge ris held zforftheibuikrof. thel heating time. Though it will be understood that 'a variety `of heating times can :be used.

6. T emperdture-tme `relation An aqueous -dispersion rof the alumina prepared as above described and containing an acid radical of the typeshown'is heatedaccording to processes of the inventionfor such a time as to eiect the growth of fibrous alumina monohydratehaving the'boehmite crystal lattice. At temperatures as low as, say, 120 C., extremely long periods 'of time are required to effect growth of the alumina "bers. Additionally at such low temperatures there canbea competitive'for'mation of other large particle'size alumina modifications suchja's prismsof gibbsite It will be `understood "that Vtherhighrer the'temperature used, the Jshorter the time required to 'effect `formation of ja `tberfor tibril ofa given character'. The times can vary widely, 'of course, as differenttypes ofproductsare to be produced, andas'diierent types of'starting-material are used. Specific examples will be given hereinafter of typicaltimesin specifcprocessesof the invention. In general, "it may be noted `that if a temperature around 220 C. is used, Vheating fora few minutes up to anrhour or so will be sufficient. On the other hand, at about 160 C., brils of increasing length are obtained with times from "say about one=half hour to about one day. Beneficial results have been obtained by heating sols for 50 to `100 hours 'at `160 `C. but such long heating times are impractical `in most `commercial operations.

"Whiletemperatures above 250 C. can be employed for verybrief periodsof time, of the order of a few minutes, prolonged heating for periods of more than about one-half hour will leadto the production of ultimate bril units which `aretoo t thick `to :be thesmost preferredproductsof the invention.

soi

. f Itis to be .observed of; the heating; times `.aridtlte tern IDA-2 j f g g f duce iibri-ls two ldimens'ions of which are Vinthecoiloldal l .range. l'j lfjnies andftemperatnres which lead to much lar/ger products :are :not preferred.;

has been converted `to the fibrousf form. The time required each system by directobservation.. l l

f Based nponthe results obtained with al variety of. proc# f fr can thus empirically bey determined? in*y each :instance for l j I y esses' ofthe invention. the .temperature andtime for`ma j y Aterials hevingaivaluei no greater than '1 canberelated empirically fas follows:` l I l l log Where.:

` l iS the ftrlctiorrtimel in minutes; l

a ijsthefimaai moiarzeoaeemrationornagqiymerizd; atte;

l miua computed' as A1203;

' X'gis the unpoyrnerized aiurninaf iconcentrationwhenthef retiree fig 1 1 reaction has attainedl .equilibriurn for lthe particular expression.

where the terms are yas stated above.

From the .above expression one can readily determine the reaction-time for a particular temperature. It will be understood that this can be varied considerablydepending upon the starting material. For materials having a value for 9 greater than lrninuteflonger reaction times can be used ata given temperature'and this can vary from the above expression by as much as, say, percent.

Using ra rapidly vdepolyrnerizable alumina, temperatures as low as, say, C. canbe used over long periods of time to effect growth of alumina bers. It is preferred to use a temperature above 120 C. though temperatures in excess of about 250 C. arenot preferred. It is more specifically preferred to operate processes of the invention at a tempcrature'between about and 220 C.

EFFECTS UPON PRODUCTS OF VARYING PROC- ESS CONDITIONS The effects of varying certain process Vconditions have already beendescribed generally above. Processes of the invention will be better understood by illustrating the eiect of varying certain conditions while holding other process conditions constant.

(rt) Concentration of alumina- !f a paradigm aqueous solution containing 2 vpercent A1203, supplied las basic aluminum chloride,.and having an A1203: chloride ion mole ratio of 1:1 isheated 4at C. for four hours a viscoustranslucent sol will be obtained.

If the concentration of alumina in the paradigm is increased, a .verythixotropic butstilltopalescenttgel will be obtained with concentrations of A1303 of 3.9.'ttndv4` :'l` rs the temperature otthelreaction on the absolute; l Q

1' f 'n jtemperaturey conditions used. and X jis the: unpolysi i j fThejabove expression can bes'ojlved for tin the follow. i; l" finaf'fnpliadifmiif c percent.. As tlleY concentration is further increased thc.-

specific surface area of` the products obtained increases in some cases as much as 100 m.2/g. in going from 2 percent A1203 to 24 percent A1203 even though the amount of unpolymerized alumina, which indicates the extent ofthe reaction, stays constant.

As the concentration is raised above about 4 or 5 percent, the products become more aggregated until at a concentration of about 16 percent A1203` a very highly aggregated product is produced. These latter aggregates in general are not ordered aggregates or tactoids but arey disordered ones. -Such highly aggregated products are not the products of this invention 'and they cannot be dispersed.

It is, however, possible to obtain a much less highly aggregated product even at such a high alumina -concentrationas 25% by raisingrthe pH to between 4.5 and 5 vprior to heating. Other expedients can be used to minimize aggregation such as the addition of organic amines in amounts sufficient to give an A1203 to amine mole ratio between 301:1 and 500:1. SuchY amines can belused as n-butyl amine and 1aminoethanol.

If the conditions of the paradigm are modified by lowering the concentration of alumina, it is found that thev'products obtained are less viscous and are more translucent. Too dilute solutions cannot be used advantageously since, as noted above, fibrils Yare not formed below alumina concentrations of about 0.05 molar.

(b)fRatz'o of alumina to acid anion 'If the chloride concentration. of the paradigm is varied, itfis4 found that above a ratio of 1:1 of A12O3zCl the product particles tend to become broader and shorter. As the ratio of alumina to chloride is increased to above the critical limit previously described a point is reached where no fibrils are formed. v

Going in the opposite direction and increasing the chloride content so' as to lower the ratio, the products do not becomewider but they aggregate intotactoids, andas the concentration lof chloride is further increased,V shorter fibers ,form aneventually, above the upper limit of the ratio specified in the empirical equation previously given, no fibersy form at all.

(c) T me and temperature Again, taking Vthe paradigm solution, the effect of varying temperature and time can be considered. Increasing the ytime of reaction over the 4 hours at 160` C. of the paradigm results in a productv having a lower surface arewand' the fibersI are more aggregated into tactoids. If .a time of lessvthan 4 hours at 160 C. is used particles are obtained with a higher surface area. For example, short particles are produced in one hour, or

even in one-quarter hour, and they havea surface areaV of up to 400 square meters per gram. In 16 hours products having a surface area `of 200 square meters per gram are obtained.

At 4 hours at 160 C. equilibrium as to disproportiona tion is obtained. At lower temperatures longer times are required. For example, below 120 C. several hun dred hours are required. In general, as the temperature is raised above 160 C. using the paradigm conditions, the products become broader and the ratiov between the length and the next smallest dimension drops below that ofthe fibrils obtained under the original conditions.

If the paradigm conditions are modified by raising the temperatures to above about 250 C. the effect of concentration of alumina becomes much'less marked. In otherwords, the products` obtained at these temperatures are about the same regardless ofthe concentration of lillumina within the ranges already indicated.

' I0 FURTIjIRI APROCESS CONSIDERATIONS f It is to be observed of the heating times and the tem'.

peratures used that it will in general be desired `to produce fibrils two dimensions of which are in thecolloidal range. Times and temperatures which lead to much larger products are not preferred. It is, nevertheless, to be understood that; assemblies of fibrils will liocculate together as shown in Figure 1 of the drawing to form fibers of alumina monohydrate having the boehmite crystal lattice which are super colloidal in size and are quite desirable products of the present invention.

As above described, a fibrous alumina can be made sim.-y

ply by heatng an aqueous dispersion ofl alumina in the presence of an acid radical under conditions as set out.- It will be understood that during the process, the alumina is converted tothe fibrous alumina monohydrate modi-` fication and is no longer polymerizable, i.e., it becomes a crystalline alumina monohydrate having the boehmiteI crystallattce. This alters the ratio-in the system of the dispersion of alumina toacid radical, the ratio dimin ishing as the reacted` alumina is transformed into the aloxane-polymerized crystalline alumina in the fibers.

The system in which fibersof alumina are lbeing formed v can be replenished with respect to the alumina dispersion by the addition of an alumina material of the Ytypes above described as suitable. This addition can be `continuous or can be effected batchwise. Thus, a suspension of finely. divided, highly hydrated aluminum hydroxide can be fed continuously.

It will further be apparent that in beginning the forma-` tion of fibrous products according to theinvention, a dispersion of fibrils which have previously beenprepared can be added to the aqueous dispersion which is used at the beginning of the process. It will also be evident that such processes as those described can be conducted` con tinuously by the steady introductionof alumina and acid into a stream vof a suspension containing the fibrous alumina monohydrate and which may or may not contain some dispersed alumina.

It will be understood that dispersions containing both fibrous aluminaand incompletely polymerized or depo1ymerized startingmaterial can be used. The formation of fibers proceeds more rapidly and at lower temperatures inthe presence of fibers of alumina monohydrate having the boehmite crystal lattice as is discussed hereafter.

The processes of the invention can be operated to ob tain various ratios between the fibrous product kand incompletely reacted starting materials. The process will have value so long as a suicient amount of fibrous alumina monohydrate having the boehmite crystal lattice is produced so that the resulting dispersion can be used for. some purpose orrcan be further treated either to separate unreacted material or convert it to fibrous material. Sometimes it will be advantageous to-.retain a small amount of the original starting material in the form of unpolymerized alumina which can function as a binder for.the fibrous particles'of alumina. Often, however, it willl be desired to remove unreacted material as will be shown hereafter.

DISPOSITION OF UNREACTED ALUMINA A ACID The acid radical remaining in an aqueous dispersion at the end of the heating step can be removed by gelling .the'aqueous sol. To do this the pH is raised' quickly to The anionfree gels` thus produced can beredispersed in distilled water to produce a solV which is essentially neutral--pH 7 to 8.5. The gelsncan be redis'persed in a desired acid, including weak acids such as those having a dissociation constant lower than 0.1 at 25 C. such as acetic and formic. The pH of th`e final sol can range downwardly from 10 and reach values as low as,` say, 1.

If the gels are to be dispersed in basic solutions care should be taken to insure that forms of alumina other than fibrous alumina andaluminum ions as [Al(H2O)+3] are absent. Partially polymerized forms of alumina such as basic aluminum ions interfere' with dispersion of an anion free gel at pH values above 8.5. t r

The absence of interfering forms of alumina can be insured as by continuing the heating stage in processes of the invention until the reaction is complete. This produces only alumina mo'nohydrate and aluminum ions. The attainment of this condition can be determined by titration. l s s v s y Thesols can be gelled and washed as above notedv and then redispersed ,by adding base in amounts sufcient to adjust the pH of `the sol to a pH up to 10. Sodium, potassium, lithiumLammonium hydroxide, 'organic amines, such as triethyl amine, tetrasubstituted ammonium Ybases such as tetramethyl ammonium hydroxide and other bases can be used.

DISPOSITION UNREACTED LUMINA When an aqueous dispersion of an alumina starting material is heated in the presence of an acid radieal as described above,"sor"ne of the alumina is not convertedto alumina monohydrate having the boehmite crystal lattice. This unconverted alumina plus any acid radical present can be regarded as a basic salt of aluminum. The strong acid radical will under certain conditions of time and temperature tie up a certain amount of aluminum and will prevent its conversion to the form of crystall line alumina monohydrate having the boehmite crystal lattice. i l When the reaction proceeds substantially to completion the dispersonproduced will, therefore, contain a le'ss basic salt of aluminum with the strong acid radical. If the conditions under which the conversion were effected did not succeed in carrying the reaction to completion there' may additionally be some unconverted starting material, or` basic salt.

The aluminum salt thus remaining can be treated as with an anion-exchanger to remove somer of the strong acid radical. The dispersion can then be heatedand `the unpolymerized alumina will be converted to the form of aluminamonohydrate having the boehmite crystal lattice. Much of the alumina appearsl to plate out as a coating on the fibrous alumina monohydrate already present in the dispersion though some can form new nuclei.

The process can` be repeated if there is' still unpolymerized alumina` present'. This can be done by again withdrawing acid radical and again heating. This can be repeated until the anion content isl as low as. desired. Theunpolymerized alumina present as an aluminum salt in various dispersions prepared according tothe in` vention will represent a minor amount ofthe totalalumij num present in the system. LThus the aluminum salt` can amount to about 30 percent by weight, computed as A1203, of the total alumina in the dispersion. Generally no more than about 20 percentof unpolymerized alumina will be present` based upon ,the total` aluminarpresenvt. lt will be seen, that there is no lower limit ,since the amount of unpolymerized` aluminum` can successively bexcome smallerland `smaller as the process `of the inven-` tion is repeated with a given sol.

QUANTITATIVE DETERMINATION F POLYMERIZED ALUMINA desirable to have analytical methods for determining the types Of ;alumina,present,andfor determining the amount ofstrongacid radical. If acetic or formicacids are. present one should, as noted later, take them into account.

By the term unpolymerized alumina .is meant alumina in anyform other ithan a crystalline form of alumina such as gibbsite, boehmite or bayerite. In general, the soluble alumina consists of amorphous `aluminum hydroxide, basic aluminum ions, and normal aluminum ions. The;quantitative,determination of allwforms of unpolymerizedA alumina can be carried out in the presence of a crystalline form of alumina such as boehmite. .v

For sols having a specific surface area of less thanabout 3 5()V m2/g. `the best procedure is to add an amount 4of 1 N HCl equivalent to 11/2 times the soluble alumina expected. For autoclavedbasic aluminum chloride solutions (with AlzOszCl 1:1 and about 2 percent A1203) this varies from about 15-30percentA of the total alumina. This mixture is then diluted to twice the volumev of the` acid originally used, allowed tostand 30 minutes in order to age the sample in 0,5 N HC1, diluted to milliliters, and titrated rapidly in small increments with 0.5 N sodium hydroxide to pH 11. When the titration data areplotted, the amount of 0.5 N NaOH addedas the abscissa and pH as the ordinate, a titration ,curve `is obtained. The amount of NaOH required between` therinection points in the curve at` pH` of 3-4 and` 7-8 is related to the amount of unpolymerized alumina present. Under `these conditions, it has been shown `that the fibrous `alumina monohydrate having the boehmite crystal Vlattice is not significantly attacked by the acid, 1

To obtain the percent unpolymerized alumina, the amount of basewrequired for the above titration from the end point corresponding to the titration of excess acid (pH 3-4) to that corresponding tothe titration of aluminum hydroxide (pH 7-8) (converted to an equivalent amount of alumina, A1O)V is determined by gravimetric n analysis and multiplied by 100.

The4 amount `of sample is chosen such that the total acidity of the sample plus the acid added is equivalent to 20-25 milliliters of 0.5 N sodium hydroxide. For examplewith a sample containing gibbsite or high surface area boehmite greater than about 350 na/g., the crystal# line alumina may dissolve to a signicant extent upon standing 30 minutes in excess acid.u 1n this case, the second sample should be run exactly as above, except that the acid-sol mixture should stand several hours before dilution and titration with base. The percent soluble aluminum may be calculated for each run and extrapolated to 0 time, provided the rate of solution is less than 5 percent of the total alumina per hour.

If the rate of solution is greater than 5 percent per hour, the above procedure cannot be used topdetermine accurately the unpolymerized alumnain the presence of crystallinealumina. In such cases, the best characterization would be the acid depolymerization rate 6 described hereafter in connection with characterization of products of the invention.

DETERMINATION oF STRONG ACID RADICAL IN soLs oF ALUMINA MoNoHYDRArE addition of base should b'e`fcnstan`t at about l milliliter per 11/2-3 minutes, being fast enough to avoid the' formationv of polymeric basic tration. The presence of .the higher polymeric cations [Anomzh' 'or rA1,(oH),1,

inthe initial sol would be indicated by an inflection at aV pH of 5.5. In the above expressions x and y indicate positive integers greater than 1 which'depend on the molecular weight of the particular polymeric cation, that is, the number of repeating units in each polymer structure.

INCREASE OF BASICITY The first step in processes for disposing of unreacted alumina, as noted above, is the removal of the strong acid radical from the fibrous boehmite dispersions above described. This is preferably effected by the use of an anion-exchanger, -or by gelation and washing as described.

Various anion-exchangers, such as those mentionedbelow, can be used for removing the strong acid radical. The anion-exchangers can be-used in partially exhausted form, or suitably buffered, so that' the pH is less than 6.

It is preferred to remove the strong acid radical using the bicarbonate form of a vstrong-base type anion-exchanger containing quaternary ammonium groups as shown in Dalton U.S. Patent 2,733,205. The anion-exchanger ean be any of those known in the art. Suitable anion-exchangers are mentioned, for instnace, in the patent cited in Dalton 2,733,205.v

The removal of the strong acid radical may be carried as far as possible. In other words, all of the radical may be't'aken from the dispersion which can be removed using the anion-exchanger. Based on the unpolymerized alumina present it is preferred vto remove the strong acid radical to an Al2'O3:acid radical (mol) ratio of kgreater than about 1:1. Actually theremoval will ordinarily go to much higher ratios, say, above 100:1.

TIME AND TEMPERATURE OF HEATING TO DIS- POSE OF UNREACTED ALUMINA IN THE PRESENCE OF FIBROUS ALUMINA MONOHY DRATE HAVING THE BOEHMITE CRYSTAL LATTICE Y Dispersions of fibrous alumina monohydrate having the boehmite crystal 4lattice from whichl the strong 'acid radical has been vat least partly removed, as above de` scribed, varethen heated. The heating converts 1unpolymerized alumina present in the system to alumina monohydrate having the boehmite crystal lattice.

It is interesting lthat lower temperatures can be used to effect the conversion of alumina to the fibrous alumina in sols of fibrous alumina monohydrate prepared as above than whenv aqueous dispersions of alumina containing little or no such fibrous alumina are heated. Ordinarily,

a temperature of at least 80 C. should be used. A tern-v perature of 100 C. is entirely practical. Higher temperatures-may be used up to, say, 375 C., but it is generally preferredto use temperatures no higher than about ,The time required at any given temperature to convert theunreacted alumina to boehmite depends upon the amount of unreacted alumina present. 'About three hours isvr required at 100 C. with a sol. containing 12 percent of unreacted alumina and having an A12O3zC1 ratio of 120:1,

aluminum ions'during the ti- Higher temperatures allow the useof shorter reaction.'

periods.

The products` produced by processes as just described are aqueous dispersions of fibrous alumina monohydrate having the boehmite crystal lattice. These are low in forms of alumina other than such alumina monohydrate and are low in acid radicals. By repeated lanion-exchange and heatingcycles the products can be made substan tially `free of both acid radicals and forms of alumina other than such alumina monohydrate.

The dispersions produced according to theprocedure just describedare stable at higher pHs than dispersions which contain an unpolymerized aluminum salt. They are also more compatible with organic solvents 'and These properties increase the rangey resin dispersions. of usefulness of the-products.

Products of lthis vinvention can be produced by processes other than those: of this invention as above described.

They can be produced by heating a basic aluminum car-.

bonate gel VVinthe presence of dilute acetic or formic ac idj under autogeneous pressures as described in detailin the copending application, Serial No. 730,024, filed April 21, 1958.

V'Briefiy the process consists of firstpreparing a very` pure basic aluminum carbonate by precipitation fromy a1um4:[Al2(SO4)3.18H2O] and a water-soluble carbonate. The mole ratioof CO3:A1 is 1.5:1 to 190:1. The gel is Washed to great purity so thatsthe SO4:Al mole ratio is less'than 0.0l:l and the cationrAl ratio is less than 0.0.4:1. The resulting basic aluminum carbonate gel is heated to to 180 C. in an autoclave to, give an aqueous sol of fibrous alumina monohydrate. The product asthus produced is a productr of the present invention though the process is not here claimed.

PRODUCTS OF THE INVENTION (1) APt'zrricle size and shape Process conditions, such as times and temperatures,

are preferred which lead to fibrils two dimensions of which'arc in the'colloidal range. Conditions which lead to much larger products' are not preferred. It is, never. theless,'to` be understood that'assemblies of brils'will occulate together as shown in Figure 1 of the drawing tolform bers which are super colloidal in size and are quite desirable products of-the present inventionl The shape of particles in aqua sols of the invention can be determined by diluting to about 0.1% A1203, or prefer# ably lower, 'with water, drying, and examining the dried material with the electron microscope.

Depending lupon the time and temperature of heating and upon the ratio of alumina to acid radical, the products obtained will be in the form of small fibrils, at least two of the dimensions of which are in the colloidal range, such as shown in Figure 2, or the products will be in theform of. aggregates or bundles such as that shown in.

Figure 1. The products 'of Figure 1 result when the particle Acharge, is such as to allow some agglomeration .andl

aggregation vof the'fibrils "to form fibersi in"which the' particles are disposed in parallel relationship along the axis of the length of theiiber.k n n Referring more particularly to Figure 2, there will-be: seen the many small iibrils which are evident uponl exf arnining` an. aqueous ydispersion.prepared according to processesA of the invention. rThis isa showing typicalr of those obtained using an electron microscope at a magnication lof about 25,000` diameters. y The brils Ias shown can occur in the products in a' random arrangei ment as illustrated Valthough they usually do not occur as products of the inventiongis that .the diameterof .the p particles of ya given product' is surprisingly `constant from` iparticle -toparticle andeach particle is of substantially uniform .diameter throughout its length. `iEven thoughv particles may differ greatly Ain' length, the diameter Aof various `particles will berelatively constant in a given single `fibrils but rather in groups of two, three, or even many more. ASuch groups form colloidal fibers.

It is to be noted that the products produced are disi 'l In other words, thefratio of .the

length ofthe iibril to the ynext'smaller dimensionhere called. width or breadth The smallest dimension yis called thickness :product prepared according to the invention.v Specific preferred typegof products will'be describedhereafter.

'Fibrils observed `in electron. micrograph fields vare ex-` amined as `to length', number, and other. characteri sticsy and, based upon experience, itis' assumed that they are typical of the fibrils present in the entire sample being examined including'brils in the aggregates. It is probable that lthe longest fibrils in a given sample have the :greatest tendency to .form Afibers by aggregation, but it is `thought that the error thus introduced in the above method. of measurement' is relatively small.

llt 'is tobe rnoted that although-the ultimate fibrils-olf;

lent diameter. f

- One can compute the equivalent diameter of particles in products of `the inventionfrom their specific surface area as determined by nitrogen absorption. The rela.- r. ftionship is as follows: f

where d is equivalent diameter in millimicrons. Equivalent diameter is the diameter of a right-circular cylinder which has the same specific surface area.

A is specific surface area in m.2/ g.

Preferably, the ratio is at least about :1. It is more specifically preferred that the axial ratio be about 50:1 to 150:1. The axial ratio can be as high as 300:1.

It will be observed that in the above discussion and throughout the application the term ibril is used in its customary sense to refer to products which under the' electron micrograph appear to be unitary as opposed to structures formed of aggregates of a number of separate members. The term fiber is used in accordance with customary meaning to include both fibrils and aggregates of brils which form `relatively long thread-like structures. The term brous Vis also used generically to refer to products which are composed of fibers which will embrace products in which the fibrils are discrete and relatively unaggregated.

It is to be noted further of the products like those of Figure 2 that at least two of the dimensions of the fibrils are in the colloidal range. Ordinarily the breadth and thickness will be of the same order of magnitude and these will be less than about l5 millimicrons but not much less than about 3 millimicrons; It is more specitically preferred that they be from about 3 to 10 millimicrons. Still more speciiically it will usually be preferred to prepare particles which have a considerably smaller diameter, around 4 to 7. t

` It will be understood that in statingthe range of cliameters, reference is made to the diameter of particles in different products. The particles in a particular product will have a diameter as described which is a figure within the range stated. One of the characteristics of products ofthe invention are believed` upon` the basis ofV "present experimental evidence to have a rectangularV cross-section, they will beconsidered asy having a circulari cross-section and thus theratio ofl length to apparent. width willlbe considered asa ratio of -lengthto equiva-` `rIf'he length ofibrils of lproducts of the invention will ygenerally be as indicated i from the ratios aboveK given.. More specifically, the products of the invention are madel 1 up ofrbrils .having 'a length from` 100 to. l, 500 1nilli" f microns. `Preferred productsr willihave a iibrillength ofk about lGOfto 700 millimicrons. y f Theredispersible, pulverulent products of the invention whichy have been 'dried' from aqueous systems ,tend to. i

havesho'rter fibrils` and slightly lower .surface area than the sols ifrom which theywere made.. kThus such prod-` ucts may have a fibril length of iup .to y1,5()0iiilli- -microns Preferred' pulverulent products have ar length rfromSOto 700 millimicrons. iThe surface area and other characteristics of such pulverul-ent products. fall within;

the limits set out herein. Using the formula above. it will be seen that at a. surface area 05200 m32! g.

such products have a diameter of-6.6S millimicrons and, at a length of 25 millimicronsyan axial ratio of 3.76zl..

lo At a surface areaof` 400m/g., the axial ratio of products with a lengthof` 25- millimicrons is`7g5 :1.

In speaking of particle size and shape it will be under stood that reference is to the average fibril particle.

Thus reference will be to average width of all particles,l

etc. lf, for example, it is said that the particles of a particular product are 5 millimicrons in diameter it is meant that the sum of the diameters of observed particles divided by the number of particles. all expressed in millimicrons, equals 5. The same diameter figure can of course be reached by observing the average length of brils in the product, and 4deriving diameter from a computation based upon nitrogen absorption. The ndividual fibrils will mostly be of about the same dimensions in products of the invention for they are surprisingly uniform. v y

As has been noted above. `there is considerable difiiculty when making an electron micrograph in obtaining a representativc area for examination. When samples are studied by drying a large droplet` and examining the entire field, not all areas of that field will be the same and there is an element of judgment as to which constitute typical and appropriate areas and which represent areas of artificially induced aggregation. `Accordingly it will often be preferred when highly accurate results are desired to use a spray-mist technique which will be described below. 4This differs from the field technique heretofore described in that instead of examining a large' measured .and a true number average obtained which is representative of the sol as a whole.

17 For the most part' the field technique has been used in the examples and specification and where the spray-l mist technique was used it is specifically identified. The limits of the invention, can be determined using either technique, but the spray-mist technique is preferred.v

a. Preparation of the sample The microscope sample grids are prepared in the nor-r mal way, such, for example, as described in Introduction to Electron Microscopy, by C. E. Hall (McGraw-Hill Book Co., 1953), on page 312 if.

A representative sample of the dispersion is, of course, required for the measurement. This is not necessarily obtainable by evaporation of a macrodroplet on the grid since only a small fraction of the total sample can be viewed. If the droplet is made small enough, however, all the particles can be seen in a single micrograph, and a representative fraction of them can be measured.

Droplets which on drying leave circular patterns of 20 microns or less are readily produced by means of a commercial nebulizer. The colloidal dispersion is diluted to 0.01-0.05 percent solids, introduced into the nebulizer, and holding the discharge end about one inch from the microscope sample grid, a stream of the droplets is directed toward the grid by squeezing the rubber bulb two or three times. Since droplets of this size evaporate almost immediately, the grid may be inserted directly into the microscope. A more complete discussion of this technique may be found in the Introduction to Electron Microscopy, by Hall on pp. 370-389, and in an article by R. C. Backus and R. C. Williams in the Journal of Applied Physics, vol.` 21, p. 11, 1950.

b.` Determination of magnification The accuracy of the measurement depends on the accuracy of which Athe magnification is known. The usual method for this determination is to include an internal standard such as polystyrene latex. This can be added directly to the sample to be sprayed but it is more economical to add it to the grid before spraying. Com

parison of the diameter of the polystyrene particles with the spacings on a standard diffraction grating has shown the particles used have a diameter of 280 millimicrons.

c. Measurement of jber length being measured. Although the more particles that are counted the more accurate will be the result, a count of 500 particles or so should give a sufiiciently accurate average particle length for most purposes.

The microscope transparency may be projected ontok a large piece of paper and the particles measured thereon by means of a rule graduated in millimeters, each fiber being marked in some way as it is measured in order to prevent omission or double counting. A preferable method, however, is to prepare first an enlarged print of the micrograph having a high degree of contrast. An 81/2 x 11 inch size using Varigam paper and a No. 8, 9 or 10 Varigam filter is quite satisfactory. Magnification under these conditions isabout 10,000-15,000 diameters. Although the lengths of the individual fibers can be measured directly with a millimeter rule, it is frequently difficult to distinguish individual fibers when they are closely aligned as in aggregates. A'more satisfactory method is to use a low-power binocular microscope,l e.g., 7-10 power. The individual fibers in most aggregates can thus be recognized and measured. It is convenient to usetwo rules (graduated in millimeters 'or preferably half millimeters) fastened together at right angles, and to mark each measured particleby means of a wax, or glass-marking pencil. In-cases in which adjacent fibers cross each other at very acute angles or contact each other at or near the ends of the bers considerable care must be taken in the identification of both ends of each individual fiber.

From the data obtained, the number average fiber length in millimeters is calculated, from which the true liber length in millimicrons is obtained by comparison with the average diameter of the polystyrene latex particles. Thus:

Fiber length in millimicrons n average length in millimeters 280 average diameter of latex particle in millimeters' (2) Surface area` specificsurface area is almost independent of the length This is true becauseV the surfaceY area yofI of the brils. the ends of the fibrils is only a small fraction of the total available surface. as a small cylinder, it will be seen from geometric con siderations that the percentage of the total surface contributed by the ends is given by the expression:

where L is the ratioof the length of the fiber to its` width or thickness.

The fibrils in products of the invention having axial ratios greater than 20:1, the contribution to the surface area by the ends of the fibrils is at the most 2.5 percent and is usually less than 1 percent of the total surface.

Measurement of the specc surface area of products of the invention therefore provides an accurate and sensitive method for ascertaining the smaller two dimen-V These will, of course, be deter? The length of` the fibrils can be determined from electron micrograph" sions of theY particles. mined as the `equivalent fibril diameter.

measurements or Yfrom measurements of streaming birefringence and this information can be combined with the" information on width as'determined by surface area, as-

suming a density of 3.01 for the alumina, to givean accurate measure of the axial ratio of the fibrils in aH product of the invention.

The specific surface area of the fibrils in sols or dispersions produced according to `the inventioncan be de'- trmined by drying the dispersed alumina from' an organic liquid medium. Thus, asdescribed below in morede` tail, normal butanol or another partially water misciblel liquid can be added to the water dispersion and a water- E* butanol azeotrope removeduntil thesystem is anhydrous. The partially water miscible liquid is then removed to leave avery fluffy dry product.l In the caseof products of high specific surface area, say, around 400 m.2/g., it

is preferred to remove the organic liquid by heatng'to above the critical point and then removing the vapors. The original structure is thereby relatively undisturbed giving a minimum of aggregation of the ultimate fibrils.

The specific surface area of the dried alumina is deftermined by drying the colloidal alumina monohydrate fibrils from an organic liquid mediumin isuch a wayas to prevent loss of area. through aggregationofgthe'fibrils as justdescribed. lThespeciiic'surface area 'ofthedried-A .alumina is determined according to the methodk of I gH l; Emmett, A New Method for Measuring the Surface The use of specific; surface areacan add considerable information concern- Thus, if the fibril is regardedV Area of Finely Divided Materials and for Determining the Size of Particles, Symposium on New Methods for Particle Size Determination in the Sub-Sieve Range, p. 95, published 'by the American Society for Testing Materials, March 4, 1941.

In products prepared by the azeotropic removal of water from fibrous alumina sols as above described, the specific surface area of the fibers as determined by nitrogen absorption will correspond to that as determined by exmining the product with an electron microscope, then measuring the fibril dimensions, and calculating the specific surface area based on the density of crystalline alumina monohydrate, 3.01 grams per cubic centimeter.

The products of this invention in general have specific surface areas ranging from around 200 to 400 m.2/g. The products having a specific surface area between about 250 and 350 m.2/g. are preferred.

The degree to which the purified colloidal alumina solution can be concentrated without becoming excessively viscous depends upon the specific surface area of the colloidal particles. Thus, a purified colloidal solution of alumina monohydrate from which most of the free ions have been removed and in which the particles have a specific surface area of around 200 m.2/ g. can be concentrated to give a stable, tiuid sol containing 15 percent A1203. In the case of products of the invention having a specific surface area of around 400 m.2/g., the concentration of alumina can be increased to about percent A1203, after removal of most of the anions, without the solution becoming so viscous that it will not pour.

Thebrous particles in alumina products of the invention are extremely stable as to their shape and crystalline form. Unlike prior alumina sols and particles they can be heated in Water to temperatures as high as 100 C. for prolonged periods, say up to a day or more, without change of shape or crystalline form.

(3) Streaming birefringence Suspensions and sols of fibrous alumina prepared according to the invention have the property of streaming birefringence.

Streaming birefringence is a property of anisometric particles, that is, particles which have one dimension which is considerably greater than another dimension or both other dimensions. Streaming birefringence is thus an empirical method of expressing shape and determining size.

The quantitative measurement of the length of the particles, the distribution of particle sizes, the Vaxial ratios, when, small, and the intrinsic birefringence of the particles can be effected by means described in the literature. Cerf and Scheraga, Chem. Reviews 51, 18S-261 (1952); Edsall, Rich, and Goldstein, Rev. Sci. Inst. 23, 695 (1952); Edsall, Advances in Colloid Science, (1942), vol 1, page 269; Joly (Trans. Faraday Soc; 48, 279-286 (1952); Barbu and Joly (Discussions of the Faraday Soc. No. 13, 77-93 (1953)); A. Peterlin, Rheology, Ed., F. R. Eirich', Academic Press Inc., N.Y., vol. 1, 1956 (p. 615).

There are four foundamental quantities which can be determined from streaming birefringence data:

(1) L, is the most frequently occurring particle length inmicrons. p

(2) P, polydispersity, is the area under the particle size distribution curve of the particles in the system. By this it is meant that in a graph made by plotting on one axis, the length of the particles in the system and on the other axis the number of particles having this length, a curve can be obtained which represents the distribution of particle sizes in the system. The area under the curve is called the polydispersity and it represents the narrowness or breadth of the distribution of particle sizes in the` system.

(3) is a measure of the breadthpor narrowness` of the particle size distribution per unit length of the particle. If the area under the particle size distribution` curve P is dividedrby the most frequently occurring particle length in the system, Lf, is obtained.

(4) mean optical intrinsic anisotropy, is a measure of the combined contributions of streaming and intrinsic birefringence. On systems containing brils of alumina produced according tothe invention, gives information on the state of aggregation and perfection of the particles.

In preparing a sample for a quantitative study of streaming birefringence, the following experimental procedure is convenient and has been followed in characterizing the products of this invention.

The sample is diluted to a concentration such that it contains 0.05 percent of A1203. The pH of the diluted sol is then adjusted to 2.0 with hydrochloric acid and the sol is agitated in a Waring Blendor for a period of 10 minutes. After this, it is put into the streaming birefringence apparatus taking care to insure freedom from dust and from air bubbles. The streaming birefringence is then determined over the gradient range of from 10- 7000 reciprocal seconds. The instrument and apparatus used are similar to those described in the Edsall, Rich and Goldstein reference, previously cited 23, 695 (1952). The above reference can be consulted for any additional information with reference to the technique for the determination of the streaming birefringence of the sol.

The determination of particle length by streaming birefringence will agree closely with the determination by` electron micrograph only if a particular sol is but little aggregated and does not aggregate during the streaming birefringence measurement. Ordinarily the measurement of streaming birefringence itself causes alignment and aggregation of fibrils so that the length Lf of fiber as determined will ordinarily be larger than that calculated from the electron micrograph. The sols of the invention are characterized by having an uncorrected value for L: which is between 200 and 2,000 millimicrons. The preferred sols ofthe invention usually have a value of L, between 300 and 800. For preferred products of the invention the value of is in excess of 3X10'2.

The product characteristics abovev described are to a considerable extent a function of each other thus specific surface area, length of bril, flbril diameter, and axial ratio are obviously interdependent. Therefore products of the invention can be characterized as to their size and shape by defining only the specific surface area and length of the fibrils.

(4) X -Ray` diracton pattern l The fibrous` alumina` monohydrate products of the invention have the characteristic X-ray diffraction pattern of boehmite. This is shown in the ASTM diffraction data` card 2-0129.

In obtaining X-ray diffraction patterns on` the products of this invention,.the samples were first dried by air drying, by azeotropic dehydration andventing fromV organic solvents, or by freeze drying. They were then mountedv in aluminum sample holders 1%1" long and wide. They were exposed to copper, Km radiation of wave length 1.54 Angstro-m units which had beenrfiltered` through a nickel filter. r

The X-rayv diffraction patterns were determined on a of the` invllln there will be found` line. positions and,

line ,intensities somewhat'unlike. the ASTM diffraction data card-above mentioned. This is to be expected since.

`of products of the invention that they, like synthetic boehmites previously produced, are of the same crystal structure as the alumina represented by ASTM diffraction data card 2-0129.

In addition to displaying the characteristic lines and approximate intensities of the ASTM data card for boehmite, products of the invention are also characterized by the ratio of the peak intensity of the X-ray diffraction line from the 020 crystal lattice plane compared to the intensity of this line in a sample of boehmite havinga surface area of less than 10 m.2/ g. It is preferred that this ratio of the peak intensities of the 020 crystal lattice plane b e less than 40:100.

Products produced according to the invention are surprisingly stable for their extremely small particle size. The alumina monohydrate fraction of reaction mixtures produced according to the invention does not have the tendency to change to other crystalline forms as do the synthetic boehmite particles heretofore known, for example, certain gels precipitated from aluminum salt mixtures.

(5) Electron dractz'on pattern The electron diffraction spectrum of fibrous alumina monohydrate placed on the electron diffraction screen from a dilute dispersion is considerably different from that ofa randomly aggregated boehmite. For example,

the three strongest lines of boehmite in the X-ray dif-v fraction pattern are missi-ng, or greatly reduced in intensity, in the electron diffraction pattern, These spacings correspond to 6.1 A., 3.17 A. and 2.34 A. On the other hand, the characteristic feature of the electron diffraction pattern is the unusually high intensity of the diffraction pattern at 1.86 A., 1.43 A. and 1.13 A., and 0.934 A.

However, lines which are missing in the electron diffraction spectrum as determined above, can be observed as fiber arcs if the specimen screen is tilted with respect tothe electron beam. The missing lines of boehmite in that case show up as characteristic arcs typical of fiber diagrams.

(6) Infrared absorption spectra Products of the invention can further be characterized by means of their infrared absorption spectra. A typical spectrum is shown in Figure 5. As at least one and usually two of the dimensions of the ultimate fibrils of products of the invention lie in the colloidal range, they do not scatter light in the infrared region.

Prior to determining the infrared spectrum of the products 'of this invention and in order to avoid the dift'iculties of working with solvents which have an infrared spectrum of their own, it is usually preferred to dry the products. This may be done either by the technique previously described of azeotropic dehydration, reaction with a suitable solvent such as normal butanol, and venting to give a fluffy, dry dispersible product; or by the technique of freeze drying which is well known in the art.

The dry, uiy product obtained by either of these techniques is then mixed with optical grade potassium bromide powder.

The potassium bromide powder is prepared by rst grinding the salt in a motor-driven mortar until it is thoroughly powdered. The potassium bromide is then screened through a 230 mesh stainless steel cloth and dried at 135 C. for 48 hours at atmospheric pressure.

The samples 4and the potassium bromide powder are Naturforsch, 8B, 66 (1953).

tons capacity, into` 12 millimeter diameter discs, in thicknesses varying from 5A@ to 2 millimeters depending on1 the weight of potassium bromide' mixture used.

The exact thickness of these discs is determined using a micrometer,` and from the measured thickness and theweight of sample introduced, the concentration in grams per millimeter of disc thickness can be computed. These frared spectrophotometer.

This technique gives optically clear wafers of potasdiscs are then scanned on avPerkin-Elmer model 21 insium bromide containing the particles of brous alumina the boehmite crystal lattice evenly monohydrate having dispersed throughout.

Since potassium bromide is virtually non-absorbing in the infrared region of the spectrum, the absorption of the infrared radiation observed with such wafers is attributable entirely to that of the fibrous alumina particles. The' use of a potassium bromide disc technique has been described by J. J. Kirkland, Analytical Chemistry, vol. 27,

page 1537, October 1955.

The following is a list of the principal infrared absorption bands for fibrous alumina monohydrate products of the invention.` The numbers shown are the approximate location of the centers of the bands in microns:

ves

It is further to be noted of products of the invention that the ratio of the optical density, or absorbance, of the infrared absorption band at 3.05 microns relative to the optical density of the band at 3.25 microns is broadly greater than 1.2:1. More narrowly the ratio will vary from 1.2:1 to 1.8:1. It is noted of samples for determination of the ratio that they should be prepared asabove described and equilibrated with the atmosphere under room temperature conditions, 30 C., 50% relative humidity, for 24 hours, tassium bromide.

RATE OF SOLUTION 1N ACID prior to blending with po- It is observed that'iibrous alumina products of the invention display a considerable resistance to acid attack and for this reason they can often be advantageouslyl greater than that of alumina precipitates prepared in the i prior art at temperatures below boiling.

The stability of fibrous alumina products of the inven tion can be examined by considering the rate at which the product dissolves in strong acid. The rate is eX- pressed as the time in minutes, 0, required to dissolve half of the product in acid, the measurements being made according to techniques described above.

While the experimental technique for the ,determina-r tion of the value of 6 for the products of this invention l is the same as that previously described for the determination of this quantity for the starting materials, the interpretation of the results obtained is usually somewhat more complicated. When weak acids y formic are present in the acids should 'be determined separately tion of the method described below.

prior to applicafor the amount of` acetic or formic product. f

such as acetic or. product the amount of such p' Titration as' described below for aluminum ion can'thenbe corrected .t acid present inthef y IDcpcndingquponfthe conditions "of teniperamr'e' auf.T time adopted during the autoclaving step,` the sols of this `tained when the percent of unpolymerized alumina, ex-

pressed asaluminum for convenience, is plotted against time.

Three factors should be noted about the plot shown iny Figure 6. These are: (l) The intercept at zero time, point X, onr the percent aluminum axis is not equal to zero; (2) the linesy are not perfectly straight; (3) there appear to be two approximately straight line portions, a

rapidly risingy portion X, Y for periods of time near the beginning 'of the depolymerization reaction, and a more slowly` rising portion, Y, Z forperiods kof time considerably after the inception of depolymerization.

The curved nature of this plot results from the fact that the dissolution of colloidal particle is a surface reaction. Ay correctiony must therefore be applied 'to the percent ofV alumina dissolved 'which willr compensate for the apparent'slowing down of the reaction as the particle f dissolves, and the available surface for reaction gradually decreases. It can be shown from'theoretical considerations that in the case ofk bers this correction is given by the expression; i

F.- (10o-percent dissolved) 1/2 wherer F is the fraction ofthe initial' surface area remaining at the time that the dissolved alumina was determined by titration.

of this correction, andy in this ligure the percent of `alu- Figure 7 illustrates the application 'y The plot of Figure lois obtained by correcting its4` slope for the depolymerization of thenuclei' and is char'- yacteristic of the products of this invention. It is from` plots such as Figure that the values of e for the products are derived. n

The value of 6 derived as above described will run y above'that of the preferred starting materials and will f usually run above 10 and preferably will exceed 50.

mina has been divided yby F. rIt will bey noted that they y lines are now essentially straight. n n

As mentioned above, various amounts of unpolyn merized alumina may'remainy at the completion of the autoclaving step. It is for this reason that the percent alumina dissolved does not equal zero at zero time. That is, the unpolymerized alumina present at the beginning of the titration will immediately show up as such even beforeany of the polymerized product has depolymerized.

This unpolymerized alumina will usually closely correspond to that which would be determined by the unpolymerized alumina titration previously discussed, although in some instances minor dilerences may result which are believed to arise from the different temperature and conditions of acidity adopted in the two determinations. In any event, and by either method, alumina in an essentially unpolymerized form will be determined at the outset of the depolymerization reaction.

If this alumina is subtracted from that determined at all other times, a plot similar to that shown in Figure 8 will result. It can be seen that this now consists of two clearly dened portions of a line going through the origin.

The more rapidly rising portion X, Y which occurs near the beginning of the reaction is thought to be due to the depolymerization of small nuclei. The less rapidly rising straight line portion Y, Z which occurs later during the reaction represents the depolymerization of the fibrous alumina monohydrate products of the invention.

If the rate of depolymerization of the products is obtained from this Y, Z portion` of the plot, the initial rapidly rising portion X, Y may be corrected for that alumina which came from the depolymerization of the fibrous alumina monohydrate product during this part of the reaction. If this is then subtracted from the total alumina determined, a plot such as Figure 9 will result, and this, represents the depolymerization rate` of the fibrous alumina monohydrate nuclei.

`The amount ofA these nuclei is determined from the intercept` of the product depolymerization line on the percent depolymerized alumina axis, point Y' in Figure. 8;.

Ordinarily the value willA not exceed about 400 minutes. The lvalue of' y(i for yspecific products will be giveny hereafter.

The products of the invention are essentially AlOOH` in the'form offbrous alumina monohydrate. As above described in connection with the alumina in the aquef f ousrdispersions used, so with the products also it will be understood that where used the term alumina signies the A1203 content. f ORGANIC-FIBROUS ALUMINA MONOHYDRATE SYSTEMS f f f The aqueous suspensions' ofthe invention, either before or after concentration, canr be transferred to organic systems. They can, for example, be transferred to aliphatic alcohols such as methyl, ethyl, propyLbutyl,

'and isopropyl alcohols. Often it is advantageous to raise the Lpli-I of aqueousI dispersions of alumina before transfer to an organic system; Thus the pH may be raised to "values of aboutr 5r or above by the use of ammonium" hydroxide or another base. In one group 'of preferred products the pH can be raised by treatment with an anion 'exchange resin. f

LThe products can also be dispersed in methyl ethyl ketone, acetone, acetonitrile, butylr acetate, and any other organic solvent or liquid which ris at least partially Water miscible. n

p The'products after the addition of a liquid which is at leasty partly water-miscible can be dried by the azeotropic ,removalk of water. partially water miscible liquid can be addedto the water dispersion and a Water-butanol azeotrope distilled until the system is anhydrous. They partially water-miscible liquid is then removed to leave a very tlutfy dry product. In the case of productsof high specific surface area, it is preferred to remove the organic liquid by heating it to above the critical point and then removing the vapors. Similarly there can be used isopropanol, methanol, ethanol, methylethylketone, methyl isobutyl ketone, toluene, and mixed systems such as ethanol-benzene and any of the numerous otherfavorable azeotropes used for forming binary or tertiary azeotropic mixtures.

Azeotropic distillation of fibrous boehmite sols produced according to the invention can additionally be used as a method of` effecting purification. Thus such a sol can be transferred to an organic liquid, preferably an alcohol, such as butanol or another of the azeotroping agents mentioned and azeotropically distilled to remove Water. It is thereafter heated under pressure and the` liquid flashed olf. The product is not only isolated as a dry fluffy powder but it is purified from the acid radical.

Why this sequence of steps effects the removal of acid radical is not understood. It may cause the formation of a. volatile compound of the acid radical and the organic liquid. For example, hydrochloric acid as the acid radical can react with an alcohol to form an alkyl chloride which would be volatilized during the venting or ashing step.

The effectiveness of the purification with respect to acid radical Varies with the surface area of the brous bochmite. At specific surface areas of 200 m."-/g. and below product having a specic surface area of 400 m.2/g`., for example, of chloride originally present was removed.

For example, butanol or another Themen-aggregated', fluffy, dry products of low anion- SURFACE ESTERIFICATION i Fibrous alumina monohydrate prepared according to processes of the invention as above described can advantageously be modified so that the particles receive a chemically attached coating of an organic material. Thus fibrous products of the invention can be coated with silicones such as trimethylchloro silane and polydirnethylsiloxane polymers. The products can also be treated with other surface coatings such as those of Sin'ani U.S. Patent 2,583,603.

vEsterification, or surface reaction, of fibrous alumina monohydrate can be effected by a process which briefly comprises treating the fibrous alumina in the absence of water with an alcohol at an elevated temperature and pressure. The product can be recovered by evaporation of the treating agent.

While the free water content of the mixture existing prior to venting is not critical so long as it does not exceed the composition of the alcohol-water azeotrope, it is convenient to reduce the water content to a fairly low value.

reduced to below about percent and preferably below 0.1 percent by weight based upon the total weight of liquid present. Satisfactory results can be obtained even if the Water content approaches the azeotrope mixture composition.

The organosol of fibrous alumina, containing more or less Water, is heated to a temperature in the range from about 100 to 300 C. though higher temperatures vcan be used below the decomposition point of the organic liquid. Corresponding pressures will of course be used. Undesirably long periods of time are required to effect esterification at temperatures as low as 100 C. so thatY in general Aa temperature from about 130 to 300 C. is preferred. y The reaction results in the elimination of Water and in the formation of a surface ester bond between the alumina and the alcohol. This is a permanent chemical bond as shown by the fact that the alcohol is held under even the most stringent drying conditions such as the use of temperatures in excess of 100 C. in a vacuum of 0.1 micron of mercury. The alcohol can be recovered by hydrolysis and determined as such by infrared.

lThe esterifying agent used is preferably a primary or secondary monohydric alcohol. Thus there can be used methyl, ethyl, n-propyl, n-butyl, n-amyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl (lauryl), n-tetradecyl (myristyl), n-hexadecyl (cetyl), and n-octadecyl (stearyl) alcohols; branched chain primary alcohols such as isobutyl, soamyl, 2,2,4-trimethyl-l-hexanol and 5,v7,7trimethyl2(1,3,3-trimethylbutyl)-l-octanol; secondary alcohols such as isopropyl, sec-butyl, Z-pentanol, 2- octanol, 4methyl2pentanol, rand 2,4-dimethyl-3-pentanol. Examples of alicyclic alcohols are cyclopentanol, cyclohexanol, cycloheptanol, and menthol. Examples of alcohols of the class having ethylenic unsaturation are allyl, crotyl, oleyl (cisu9-octadecen-1ol), citronellol, and geraniol. v

The alcohol used can have any number of carbon atoms but it is generally preferred that it contain ,no more than 18 carbon atoms.

For ease of recovery, the water content ought to be -After completion of the esterification reaction lthe fibrous alumina can be separated from the alcohol in any I It' can be removed by filtration or by Where high temperatures are used, the alcohol or alcohol-water azeotrope suitable Way. simple-drying as in a vacuum oven.

can be removed by venting the autoclave in which the reaction is conducted.

The percentage of esterification achieved can readily be determined by analyzing the product for carbon and hydrogen and determining surface area byvcalculation from the dimensions of the particle as determined by an electron micrograph or by determination of the surface i area by nitrogen adsorption as previously discussed.

Alternatively, the percentage of coverage of the surface by alkoxy groups can be determined by hydrolysis of the surface ester linkages followed by analysis of the hydrolysate for alcohol content. The number offsurface alkoxy groups per square millimicron of fibrous alumina monohydrate surface will normally range from 0.1 to approximately 4.

Readily dispersible products which can be mulled into Water or other liquids to form usable suspensions are very important products of the invention.

preferred products are dry, that is, they do not contain enough liquid water to appear wet or pasty on visual inspection.

The dispersibility of dry products of the invention in lvarious media can be affected in various ways suggested such agents which will provide an organophilic coating. Dispersibilty in aqueous systems can similarly be aided by treatment with the relatively short chain alcohols such as methyl, ethyl, and propyl alcohols. Dispersion of dry products of the invention in water is also aided by conventional surface active agents. The presence of minor amounts of soluble salts such as sodium chloride and sodium nitrate in the process for making the alumina fibers also results in products of good dispersibility. Dispersibility in both aqueous and organic systems can also be aided by drying the products in a Way which will leave them light and relatively unpacked. Thus they can be spray dried or freeze dried.

COMPOSITIONS CONTAINING FIBROUS ALUMI- NA PRODUCTS OF THE INVENTION In addition to the stabilizing acid ion above described there can be introduced into the fibrous alumina solsy other stabilizing ions with a dissociation constant less than 0.1 at 25 C., especially after the stronger acids have been removed. For example, there can be used acetic acid, formic acid, sulfamic acid, and carboxylic acids in general, or their salts. Sulfuric and phosphoric "acids, or their salts, can be used. The selection of a particular acid radical, and whether the acid itself is used or one of `its salts, will depend upon the specific use to which the product is to be put.

Alumina monohydrate fibrils, aggregations of fibrils,

.and alumina fibers made up of aggregations of fibrils can be incorporated in the various compositions in which they are to be used depending upon their nature. Often the fibrous alumina can be added in a dry form and simply milled or mixed into the composition. Sometimes it They can be pressed into pellets or other readily handled form, but all are pulverulent so that they are readily redispersible. The

orit can be water, depending upon the particular composition. p

` For example, if the fibrous alumina is to be added to` a wax emulsion containing a hydrocarbon oil, the brousV alumina can be transferred to a compatible, and pref-` erably the same hydrocarbon oil prior to addition to the wax emulsion. It is generally preferable in Such a case` to first transfer the fibrous alumina to a water miscible. organic solvent and then to the water immiscible solvent.. Similarly, if` the brous alumina is to be added to an aqueous wax emulsion, it can be added most readily as an aquasol.

Fibrous alumina products of the invention, preferably ina dry form, can be mixed with one or a combination of dry lubricants such as graphite, molybdenum sulfide talc, and powdered mica. Compositions of this type can be used for lubricating the surface of metals during form-v ing operations, including rolling, stamping, drawing, andA die casting. The lubricant composition can be applied. to the metal prior to the forming operation or applied to the surface of the forming equipment. `It can be ap plied to the inner Surface of molds into which metals are cast.

Fibrous boehmite, that is, fibrous alumina monohydrate having the boehmite crystal lattice, either dry or in a suitable organic solvent, can be mixed with volatile oils; for example, kerosene, gasoline, and naphtha, or with organic solvents such as benzene, carbon tetrachloride, etc. Such mixtures can be applied to surfaces for the` same purposes as are above set forth for dry mixtures. In this case, the evaporation of the volatile component leaves the fibrous boehmite as the lubricant on the treated surface.

Oils that can be used to make greases by admixture with fibrous boehmite as a thickener are the hydrocarbon oils, fiuorocarbon oils, silicone oils, vegetable oils, stearic acid and other long chain fatty acids, polymeric esters, diesters, such as di-Z-(ethyl hexyl) sebacate, cottonseed oil, whale oil, polyethers such as polyethylene oxide oils, castor oils; in fact any animal, vegetable or mineral oil can be used, and also synthetic chemicals having typical oily characteristics. Fibrous boehmite can also be used with hydrophilic oils, such as polyethylene oxide oils. Fibrous boehmite can also be used in any oil composition containing a water immiscible oil.

The proportion of fibrous boehmite used as a thickening agent will depend upon such factors as the character and viscosity of the oil, the nature of the grease which it is desired to produce, and the exact nature of the boehmite itself. While it is sometimes feasible to use as high as 70% of the boehmite in the grease composition, it will ordinarily be found that a smaller amount is adequate.` The action of the boehmite is pronounced even with small amounts, and excellent greases have been made using from 3 to 15 of the boehmite.

The mixing of the oil and fibrous boehmite is carried out in any manner which has heretofore been used for introducing other non-soap thickeners into oil. Fibrous boehmite can be used in conjunctionwith conventional soap-type thickeners, such as sodium or lithium stearates or hydroxy stearates.

When added to oils in smaller quantities than sufficient to thicken the oils to greases, say 0.1 to 3%, fibrous boehmite gives compositions which show much less change in viscosity with temperature than does oil alone over a wide range of temperature.

The water resistance of these greases can be improved by methods known in the art, e.g. ULS. 2,599,683 and 2,583,603. The fibrous boehmite alumina can also be coated with a monomolecular layer of a longchain soap in order to improve its dispersibility in a hydrocarbon oil medium and the Water resistance of the' resulting grease.

Fibrous boehmite can advantageously be included' in cosmetics, either as adr'y product or dispersed in a` suitable organic liquid. Greases, salves, creams, cosmetic emulsions,.hair oil, lipstick, face powder, anti-perspirants, deodorants, and theatrical make-up materials can be improved by incorporation of fibrous boehmite, in amounts of, say 1' to 20%. In aqueous formulations of such products, fibrous boehmite is especially useful as a thickening, emulsifying, dispersing, and suspending agent.

Fibrous boehmite can also be used as an adsorbent or carrier for enzymes, viruses, alkaloids and various antibiotics and vaccines.

Because of its film-forming nature fibrous boehmite is useful in compositions for treating peptic ulcers, alone or in combination with conventional formulating agents. The fibrous alumina monohydrate products of the invention can be included as a thickener in food products, for example, in ice cream. The fibrous boehmite can be dyed with appropriate food dyes and added to food products to effect both thickening and coloring.

Inks, such as printing inks for letterpress, lithographie or gravure type processes, are improved by incorporation of fibrous boehmite asI a dispersing, thickening and extending agent. Because of the film-forming properties of fibrous boehmite, superior adhesion to papers and fabrics is obtained, and clarity and definition of the print is improved. With colored inks in particular, small amounts of fibrous boehmite enhance brillance, brightness or depth of shade. Because of substantivity to fibrous boehmite, pigments and dyes may, if desired, be adsorbed on the surface of the boehmite fibrils prior to incorporation in the ink formulation.

Fibrous boehmite is useful at concentrations of 0.5 to` 25% as a thickener, dispersant or emulsifying agent in aqueous fioor wax emulsions or pastes utilizing conventional components such as carnauba, candellila, beeswax or synthetic waxes, and natural or synthetic resins. On drying, improved leveling and polishing properties may be realized, with better hardness, scuff resistance or anti.- slip properties.

Fibrous boehmite can be incorporated into elastomer products in amounts of 1-30% by Weight to improve strength and/or abrasion resistance. This can be done at any point in their manufacture, including the original formation of the polymer. Generally, however, fibrous boehmite will be incorporated by conventional milling and compounding techniques commonly employed` with other fillers. The elastomer in which fibrous boehmite is incorporated according to this invention can be any rubber-like polymeric material. The term elastomer is a general descriptive term for this class of products and may be regarded as an abbreviation for elastopolymer or elastic polymer. (See Advances in Colloidal Science, vol. 2, 1946, p. XXV.) As here used it may be understood to cover the high molecular elastic colloidal natural caoutchouc, aswell as synthetic rubbers and rubber-like materials such as Neoprene, butyl rubber, and the styrene-butadiene copolymer known as GR-Stypes, butadiene-acrylonitrile copolymers, polybutadienes, and the polyisoprenes.

It will be noted that the invention is applicable to diene elastic polymers as a class. Fibrous boehmite can also be used with `chlorosulfonated polyethylene, and fluorocarbon rubbers, polyester rubber, silicone and polyurethane rubbers.

A finished sponge or foam of natural or synthetic rubber or other elastomer can be treated with an aqueous boehmite sol, say a 2 to 15% A1203 sol in order markedly to improve the load carrying capacity ofthe foam.

The normally hydrophobic surface of a polyurethane type sponge can be made hydrophilic by treatment with 0.5 to 10% of a fibrous alumina monohydrate product of the invention followed by drying of the treated sponge at 50 to 110 C.

Fibrous boehmite can be used in plastics in manners analogous. and in amounts comparable, to those debeing formed into threads or fibers.

sequently slit into ribbons or threads.

scribedn for* the4 in rubben For example, fibrous boehmite can be` nse'd as a reinforcing ller in making plastic films, coatings, paints; adhesives, or other plastic articles. When incorporatedy into the organic polymer prior to extrusion into sheets, tubing, or lacing, the fibrousv boehmite improves the tensile and/or impact strength. Among organic plastic materials which are especially benefited by incorporation of 1--40% of fibrons boehmite are melamine, urea, and phenolformaldehyde plastics, polyester resins, alkyd resins, epoxy resins, polyvinyl acetates, polyvinyl acetals, polyvinyl alcohols, polyethylenes, polyacrylic esters, polyacrylonit'r'ile,l silicone plastics, regenerated cellulose such as celoplian'e and rayon. Fibrous boehmite may be mixed with aqueous dispersions of such polymers or it may be added inthe process of their formation, as for example during 'the polymerization in the case of vinyl polymers. In aqueous emulsion or 'dispersion polymerizations, it rna'yr serve as a dispersing or nucleating agent when used alone for in the presence of other more conventional dispersing or emulsifying agents.

. y,It is Itobe noted that fibrous boehmite can be used for the surface treatment of practically any type of article.v Thus painted surfaces, solid plastic objects, paper, rubber articles, textiles and upholstery and other .fabrics including pile fabrics such as rugs can be treated.

The surfaces Vcan betreated byapplyin'g a dried fibrous lliioehrnit'e by .vigorous rubbing, or more easily by applying an aqueous orl organic sol of the boehmite.

c Fibrous boehmite can be applied also to the surface of fibers, not only synthetic fibers such as nylon .polyamides Orlon acrylic, Daeron polyester, cellulose acetate and rayon, but also natural fibers such as wool,l cotton, silk, raniie, hemp, alpaca, camelhair, fur, feathers, goathair, horsehair, and animal bristles generally.A Not only can the fibrous boehmite be applied 4as Ia surfaeereoating on individual fibers, but can also Vbeirnpregnated onto the surfaces of twisted threads and 'wovenv textiles, as noted above. The treatment of such surfaces prevents vthe deposition and retention of soil and also beneficially modifies the surfaces in respect to the pick-up of staticA electricity.

, A'Fibrous boehmite can be incorporated into organic jfibers ,prior to drawing and spinning, in amounts ranging from laptralce up to high percentages, say 0.1 to 50%, vdepending upon the effects desired. Fibrous boehmite ucan be yincorporatedinto fibers of the following organic 'polymeric types: nylon, Orion acrylic liber, cellulose ljacctate, Ypolyvinyl chloride, polyethylene, rubber, ffTelion, polytetrauoroethylene, Daeron polyester fiber andjall lsynthetic organic compositions capable of i The fibrous boehmite may be incorporated intothe fibers in various ways, including dispersion in the fiber melt, dispersion v Yin the polymer latex prior to forming threads, dispersion `in the polymer solutions priorto wet or dry spinning, or incorporation into polymer sheeting which is sub- In small amounts. the fibrous boehmite improves the dyeing and drawing characteristics, and when incorporated in the form of aggregates sufiiciently large to promote the scatterjngofllight anda certain degree of surface roughening, fit acts as a delusterant and improves the antislip prop- "erti's'ofthe fiber. At relatively vhigh concentrations, for example 5'-25%, fibrous boehmite produces desir- "able increases in modulus, tenacity kor elongation, and

1th some polymers improved hihg temperatures characteristics. A l

,11n any of the foregoing uses and in many others, fi- "brous boehmite ycan be used as a lake subsstrate and "'added'to any of the various compositions described.

Forie'xample, the fibrous boehmite can be dyed while ,in water solution or in an appropriate organic solvent 'andthedy'e'dvbers'can vbe included inplastics which "are sbse'qiiently'tobe formed into nlms, threads, -or

30 the" like; Similarly, they' can be used in various cornpositions where it is desired to impart color as well as the valuable physical properties resulting from the use of fibrous boehr'nite.

Fibrous boehmite when ignited above about l000 C.- gives a fibrous form of anhydrous alpha-alumina. This novel fibrous alpha-alumina is useful in the manufacture of refractories by combining it with finely divided metal oxides and firing. Pure alumina sintered bodies can be prepared by compressing the dehydrated fibrous boehmite before firing, preferably with a small amount of an organic binder which burns out the mass. Thus, organic soluble metal salts such as aluminum acetatae or metal stearates may be used.

Porous are cellular ceramic bodies with higher cornpressive strength can be obtained by the incorporation of dehydrated fibrous boehmite. The ceramic body can be made by including in the original mix carbonaceous materials which are later burned out. Or destabilized hydrogen peroxide which evolves oxygen gas during heating can be used in place of the carbonaceous material.

Powdered metals are improved by incorporation of fibrous boehmite. The fibrous boehmite can be mixed with the powdered metals to form a thin insulating coating on the metal particles. Metal compositions containing 0.01% to 10% by weight fibrous boehmite, after compression and sintering, contain a unique distribution of the fibrous boehmite throughout the metal structure,

providing improved strength, particularly in compression. With the incorporation Aof large proportions of fibrous boehmite such as 10% to 70% by weight, the sintered metal products show lower mechanical strength but also lower thermal conductivity, which is important, particularly in the cases of the metal products to 4be exposed to very high temperatures.

Fibrous boehmite can also be used in the production of 'the newer types of cermets or metal-ceramic composite Structures, in conjunction with powdered metals such as chromium, nickel, cobalt, iron, etc. Such cernposite structures can be made by intimately mixing the finely divided metal and fibrous boehmite alone or in combination with other refractory oxides such as beryllia, chromia, magnesia, etc. and compressing or extruding `at a` high temperature in an inert or reducing atmosphere.

Fibrous boehmite as a thin, carefully dehydrated film on metals yields a thermal and electrical insulating pro- -tective coating. Thus it can be `applied from a 2 to 1,0 percent sol to aluminum, which preferably has been cleaned to remove oxides. It can then be heated in boiling water to complete the protective coating.

Extremely thin films of fibrous boehmite can be used to promote the adhesion of two dissimilar materials by providing high surface area anchor points. For example, fibrous boehmite films can be used to improve the lbond between a paint film and a metallic surface, between various polymer films such as Mylarf copolyester of ethylene glycol and terephthalic acid, or regenerated cellulose and other substrates such as metals, glass or other films.

Binders for fibrous ceramic products such as rock wool and glass fibers are improved through the incorporation of 2fto 20% by weight of fibrous boehmite. The fibrous boehmite is highly substantive to surfaces containing silica and can act as a binder itself. For example,

` phenol formaldehyde resin emulsions can be mixed with 'fibrous boehmite or alkyd resin emulsions and yapplied to the fibrous inorganic material to obtain strong bonds residual alumina forming the body of the bond. For

ythis application a distribution of particle sizes of fibrous "boehmite is preferred but not necessary.

yFibrous "boehmite can be used asa base for cracking catalysts and `other alumina catalysts. The fibrous boehmite sols can be mixed with a silica sol toform a Agel which after drying and dehydrating can be formed into pellets or mats. For example, chromia supported on a dehydrated fibrous boehmite or ya silica alumina co-gel support can be used as a catalyst for the low pressure polymerization of lolefins, such as ethylene. The fibrous boehmite can be formed into suitable beads or particles together with other catalytic agents in conventional manner.

Films of fibrous boehmite can be used as such or in combination with minor ramounts of organic and in# organic materials to modify the properties of fibrous boehmite films. For example, the films can be modified with polyvinyl alcohol, Telion polytetrafiuoroethylene, Mylar polyester film, polyethylene, and polyvinyl fluoride.

Inorganic materials, especially those of a iibrous or i plate-like nature, can also be used to modify fibrous boehmite films. For examplesmall amounts (e.g., less than 1020%) of minerals such as bentonite, attapulgite, wollastonite, halloysite, kaolin, talc, exfoliated vermiculite, mica, especially waste mica splittings, asbestos, etc.,

can be used. Synthetic librousr materials in smallr amounts such as glass fibers, Fiberfrax ceramic fiber,

finer fractions of rock wool, etc., also can be used.

Fibrous boehmite is several times more efficient rthan alum in resin-alum sized papers, and at the Sametime is able to cause an increase inr the spreading of ink on the papers.

Fibrous alumina monohydrate is quite useful when in corporated either as afiller or asin situ mixtures with tobacco in cigarettes. At l to 10% concentration the products of this invention filter out a high percentage of the 'tars' and other undesirable constituents of tobacco smoke and give a mild, free-burning, easily drawing cigarette;

In order that the invention may be better understood, the following specific illustrative examples are given in addition to the examples already given above:

EXAMPLE 1 A basic aluminum chloride solution was prepared according to the teaching of U.S. Patent 2,196,016 as follows:

241.4 grams of aluminum chloride hexahydrate were dissolved in 1,000 grams of water. 27 grams of aluminum metal powder was then carefully added to the solution of aluminum chloride. At the outset of the aluminum addition, the solution was heated to about 65 to initiate the reaction and then the remainder of the aluminum was added cautiously while agitating the mixture vigorously. Once started, the reaction proceeded exothermically with the evolution of hydrogen.

The basic aluminum chloride solution obtained contained by analysis 8.00% A1203 and 8.43% Cl. The alumina molarity calculated as A1203 was 0.90. The solution had a specific gravity of 1.1388 at 25 C., a pH of 2.96 and a specific conductivity of 96,600 micro mhos/cm. The value of 0 was under one minute.

Ten volumes of the above solution was diluted with water to 100 volumes. The diluted solution had an alumina content equivalent to 0.09 molar as A1203, and a molarity of the acid radical, chloride, of 0.271. The pH of this solution was 4.01 and the specific conduc tivity was 21,340 micro mhos/cm.

The solution was heated in a sealed glass container at 160 C. for 16 hours. During this heating, fibrous boehmite precipitated in the initially water clear solution. The fibrous boehmite was readily dispersed upon gentle shaking. The resulting suspension had a pH of 1.78 and a specific conductivity of 27,100 micro mhos/cm.

Examination of the boehmite by means of an electron microscope at a magnification of 25,000 diameters showed that the fibrils produced had in large measure `artialratios of atleast 50 to 300 and diameters of `5` to 10 millimicrons. `Some parallel algrnnent of the fibrils had occurred to produce aggregate bundles.` `The prod- -uct appears as in Figure 2 of the drawings. y

` EXAMPLE 2 A basic aluminum chloride with a Vmolar ratio of' Al2O3/Cl=2/3 was prepared by a method similar to that described in Example 1, viz.,24l.1 grams of aluminum chloride crystals were dissolved in 1,000 grams of water, and then to this solution 81.0 grams of aluminum metal powder was added uniformly and slowly until all the metal had dissolved `and nomore hydrogen was evolved. As before, the solution was pre-heated` to f about 50-60 C. to initiate the reaction, after whichthe heat was removed `but the temperature remained at about 80-90 C. throughout the reaction.

` The product of this` reaction was water clear and slightly more viscous than `the corresponding solution obtained in Example 1. Analysisof this solution showed that it contained on a weight basis, 15.4% A1203 and 8.05% Cl which corresponds toa molarratio Al-Oa/Cl of 0.67/ 1.00, and an alumina molarity of r1.86'. The f density of this solution at 25 C. was 1.2244 g./crn.3, while the pH and specific conductivity were 3`.ll and 77,600 micro mhos/ cm., respectively. The value rof 0 was f under one minute.

Five volumesof this solution were diluted up to a total of 100 volumes with distilled water andthe mixture shaken. The concentration of alumina in this solution wasf0.093 molar (as A1203) rwhile the chloride molarity was 0.140. This water clear solution hada pH of 4.08. It was then placed in a sealed glass container and autoclaved for tive hours at 160 C. Upon cooling, the material removed from the autoclave contained a white precipitate which, upon electron microscopic examination and X-ray diffraction examination was shown to contain very long hair-like fibrils of aluy.mina monohydrate having the boehmite crystal lattice.

Practically all of these fibrils of boehmite alumina had formed tactoids by a sidewise parallel alignment, and these small tactoids had, in addition, aligned themselves in an endtoend fashion Ato product very long fiber `bundles of boehmite alumina as illustrated by Figure l.

The fine hair-like ultimate fibers or fibrils of alumina were about 3-10 millimicrons wide and about 0.S-l.0 micron long and were clearly visible `at a magnification of 25,000 diameters; the individual tactoids or sidewise aggregates of ultimate fibers were about 0.25-`0.5 micron wide and 0.5-1.0 micron long;,the end to end aggregated tactoids were about 0.25-0.5 micron wideandl as much as 5-10 microns long.` No other crystalline shapes or other crystalline forms of alumina could be detected in the sample. The product appears as in `Figure 1 of the drawings. r

The pH of the suspension of this readily dispersible precipitate was 2.03. This suspension would easily pass through a filter paper when vacuum filtered. When dried down, this suspension formed a thin film which remained coherent and translucent when removed from the substrate.

EXAMPLE 3 l `A solution was made by dissolving in 98 parts by weight of water 2 parts byweight of a solid water soluble basic aluminumchloride product. Chemical analysis of the solid basic `aluminum. chloride showed that it contained 45.72%.Al203, 16.58% chloride, and 33.27% volatile matter calculated as water, thus having a composition corresponding to an Al2O3/chloride` ratio of 0.96/1. X-ray` difiraction studies showed `the watersoluble solid to be amorphous. .The dried aqueous solution did not contain discrete structures or particles on solution was heated in a sealed glass container for 16 hoursat 165 to'l70 C; There was thereby obtained a stable opalescent alumina sol having a pHl of-2.32 and a specilic conductivity of 15,7 micro mhos/cm.

Examination of the sol by electronY microscope at a magnification of 25,000 diameters revealed that the sol 'contained long fibrils having axial ratios of at least 50:1 'and breadths in the colloidal'range. The brils were identified as having the boehmite crystal lattice by X- 'ray diffraction. The product appears at 50,000 diameters as in Figure 3 but is slightly broader and slightly longer.

A fibrous alumina aquasol, the yfibrils of which have a specific surface area of about 200 m.2/g., was transferred to normal propanol by azeotropic distillation. To this there was added a small amount of alizarin yellow GG until the fibrous boehmitel had a brilliant yellow color. The normal propanol was removed. The yellow fibrous boehmite powder together with about an equal weight of undyed fibrous boehmite was incorporated in a total amount of 10% by weight in polyethylene molding powder. Brilliant yellow films which were quite clear' and transparent were obtained. Similar colored of Example`4 was diluted to 100 volumes With'distilled water. The molarity of alumina in the resultin'grdilute solution was 0.0971and the chloride molarity was 0.0934. The pH' and specific conductivity of this clear solution were y5.20 and 6,190 micro mhos/cm. respectively. The valve of 0 was under one minute.

A portion of the dilute solution was heated in a sealed glass container for 16 hours at 160 C.

rl`he product resulting from the heating treatment was a stable, sediment-freed, opalescent, hlm-forming, thixotropic alumina sol, the alumina beingl in vthe form of fibrous alumina monohydrate having the boehmite crystal lattice. The product yhad a pH of 2.51 and a specific films `were also obtained using a variety of dyes in Vthe same way.

EXAMPLE 4.

242 grams of aluminum chloride hexahydrate were dissolved inv 1,000 grams of water. This solution was heated to about 50-60" C. and then 135 grams of alumi- 'num metal dust were slowly and uniformly added with good agitation. After once starting, the reaction proceeded exothermically with the evolution of hydrogen and the temperature remained at about 80-90 C.

The resulting solution was almostl water clear but perceptibly viscous. Analysis of the solutionshowed that it contained 22.73% A1203 and 7.58% Cl which corresponds to an Al2O3/Cl mole ratio of 1.04/1. The density of this solution was 1.322 g./cm.3 at C. The pH and specific conductivity were 3.81 and 53,400 micro mh'os/cm., respectively. Ten volumes of the solution prepared as above was diluted to 100 Volumes with distilled water. The resulting clear solution had an alumina concentration of 0.295 molar, a chloride content of 0.283 molar, and a pH of 4.51. under one minute. v

This solution was heated in a sealed glass container at 160 C. for 5` hours. The heatingv in the sealed glass container converted the clear solution to a translucent, semi-rigid, spontaneously birefring'ent gel. By vsimply di'- luting with distilled -water and mild shaking by hand, this .gel readily reverted to a stable opalescent sol exhibiting streaming birefringence. The sol lcould be filteredA unchanged.

Thealumina in the redispersible gel of this example `was alumina monohydrate having the boehmite crystal f lattice. Electron microscope examination at 25,000 diam- 'eters showed that only iibrous aluminaw'as present, most vof the fibrils having a breath vin "the orderv of 5 to 10 'millimicrons and an axial ratio of about 200: 1. In some cases, 3 or 4 of the ibrils were aggregated sideby side vto product longer aggregate fiber lengths. The product appears at 50,000 diameters as in Figurey 3 of the drawings -but they are somewhat shorter and narrower.

It .is noted that, particularly with products asfine as 4those of Figures 2jand 3, it is difiicult to state the Vexact breadth and, often, even the exact length from an examination of the electron micrograph. After an examination of these products by nitrogen absorption and by streaming birefringence as previously noted, lone is enabled to interpret the electron micrographs more accurately and to make more exact .calculations of the physical dimensions of the products seen. v Y

EXAMPLE 5 ride `solution prepared as described "in the 'first paragraph The value of 'was conductivity. of 10,650 micro rnhos/cm. The product appears at` 50,000 diameters as in 'Figure 3 of the drawings, but is somewhat broader..

A portion of fibrous boehmite sol prepared generally as above but containing particles of 7 millimicrons diameter and of lengths from a half to one micron, was transferred to normal butanol by az'eotropic distillation at constant Volume. This solution was placed in anautoclave, heated to 3007 C. and vented to remove the alcohol and acid radical. The product was then vacuum dried to remove residual butanol.

The white iiutfy .powder thus produced was incorporatedV in a grease prepared from'a mid-continent solvent treated petroleum oil (viscosity300 SUSvat F.) by milling the oil with 13% by weightof the fibrous boehmite on an ink mill.

The grease obtained was clear andhomogeneous. Its consistency was greatly increased by the inclusion of the fibrous boehmite.vr The grease was stable to comparatively long exposures at elevated temperatures, say up to F.

A clear and homogeneous grease can also be obtained by milling a mixture of 13% of a dried fibrous boehmite -as yjust described with a silicone oil having a viscosity of 100 centistokes at 25 C. v

vFibrous boehmite can be-incorporated into natural rubber employing techniques v'common in'A the art. For example, fibrous boehmite dried by azeotropic distillation, as above described, canbe compounded into a formulation as follows: v

Ingredients: vParts by weight Smoked sheet 100. Zinc oxide 5.,v Phenyl a-naphthylamine 1. Fibrous boehmite 60. Stearic acid 1. Sulfur As indicated. Benzothiazyl disulfide 2. Tetramethyl thiuram disulfide' 0.1.

The fibrous boehmite'can be any of those herein described, and specifically there can -be used a fibrous boehmite dried by azeotropic distillation and having libril diameters of about 5 millimicrons and fbril lengths of about one-half to one micron., The blend can be cured according to conventional methods to give rubber of increased tensile strength. w f vvA granular, solid polyethylene molding powder, such as ,Alathon1 polyethylene resin made by Du Pont, can be ymodified by milling thereinto an .equal weight of a dry fibrous boehmite of the type above described. Thus there can be used a fibrous boehmite having a surface'area of 200 m.2/ g. and the boehmite fibers can have an average diameter of about 7 millimicrons and lengths of from one-half to one micron.

.The polyethylene containing fibrous boehmite was molded iuto a thin film. The film was .clear and it was stiffer than a similar film not containing fibrous boehmite. The film so treated ,was immersed in a dilute acid dye solution eosine a'redv dye,V and the filmr dyed a deep, brilliant red. `Without the iibrous boehmite present a film of polyethylene could not thus be dyed. 

1. FIBROUS ALUMINA IN THE FORM OF FIBRILS HAVING AN AVERAGE LENGTH IN THE RANGE FROM 100 TO 700 MILLIMICRONS, THE REMAINING AVERAGE DIMENSIONS BEING IN THE RANGE FROM 3 TO 10 MILLIMICRONS, THE AXIAL RATIO BEING FROM 50:1 TO 150:1 AND THE PRODUCT HAVING THE FOLLOWING PHYSICAL PROPERTIES: 